1. Field of the Invention
The present invention relates to a stage system, a lithographic apparatus including such stage system and stage control method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Increasing demands on throughput of the lithographic apparatus, e.g. on an amount of wafers to be processed by the lithographic apparatus in a certain time period, require increasing speeds and accelerations of stages such as a substrate table or a mast table. Furthermore, demands on resolution and accuracy of the pattern to be projected by the lithographic apparatus onto the substrate increase, which translates into a tendency to increase a size of the mask, in combination with an increased demagnification factor of the projection optics, the increased size of the mask to be able to provide the required level of detail on the mask. On the one hand, requirements as to a high scanning speed necessitate to keep a weight of a stage as low as possible, while on the other hand, the stage need to be constructed such as to obtain a high stiffness, to avoid occurrence or excitation of resonance modes of the stage which tends to result in massive constructions. Even further, to be able to achieve a high internal stiffness, use is made of materials having a high stiffness, unfortunately having a small relative damping, resulting in a long time before internal vibrations have settled.
Inspecting some of the design criteria more in detail, servo disturbance rejection and tracking performance (settling behavior) may be improved by increasing bandwidth of a closed loop control system by means of a feedback controller. Internal structural resonances of the stage, which are always present, impose severe constraints on extent in which the closed-loop bandwidth can be increased. For this reason, considerable effort is put by control engineers during design of the stage into optimizing mechanics in such a way that these resonance frequencies are as high as possible, allowing a high bandwidth. Generally, based on specifications of the process, a required minimal bandwidth is selected. The mechanical design is then optimized, providing all resonances above this bandwidth. In general, this may imply stiff coupling between actuator and sensor positions, which may result in a relatively heavy construction. In order to design stiff constructions, high E-module materials are used, which are intrinsically badly damped. Due to variability in production of the stages, a controller has to be robust against variations in the plant dynamics. In general, it is desired to have a same controller design is used for all produced items of a particular stage, which also induces some conservatism, which again limits performance.
Throughput enhancement or in other words higher accelerations and small settling times may generally have a negative effect on the stage accuracy and thus overlay. Higher accelerations may cause higher internal dynamic vibrations (or deformations) of the stages, which may be intrinsically badly damped, possibly resulting in a deterioration of the stage accuracy as the settling time decreases. Furthermore also the disturbances on the stages and the environment (or the “silent world”), which are induced by the movement of the stages themselves due to cross-talk, may increase (e.g. the lens, immersion) which may also result in a deterioration of the stage accuracy.
Since vibrations and disturbances may become limiting for the stage accuracy and thus overlay as the throughput increases, it is desirable to solve or at least alleviate the above limitations and conflicting requirements.
Thus, given the high requirements on the throughput of the lithographic apparatus, conflicting requirements come into existence, which appear to result in an upper limit to the performance that can be achieved.